Structure Of Graphene Nanoribbons Research Articles

Energetics and electronic structures of N-doped graphene nanoribbons with pyridinic and graphitic edges

Using the density functional theory, we study the energetics and electronic structures of graphene nanoribbons of which edges are substituted by N atoms. Our calculations showed that the edge N atoms with pyridinic structure mimic the π environment of C atoms, participating the π electron network of C atoms. In contrast, N atoms act as impurities for the π electron network of the nanoribbons when they are terminated by H atoms, leading to versatile edge localized state those are hardly synthesized using C and H atoms. The total energy of N-doped nanoribbons is the same as that of the pristine graphene nanoribbons, irrespective of the edge hydrogenation, and gradually decreases with increasing nanoribbon width. We also found that the edge hydrogenation decrease and increase the stability of the N-doped graphene nanoribbons with armchair and zigzag edges, respectively.

Modeling of lightly-doped drain and source contact with boron and nitrogen in graphene nanoribbon

Graphene, a monolayer carbon atoms arranged in hexagonal honeycomb lattice possesses impressive electronic properties. It is utilized as channel, source and drain contact in graphene nanoribbon field-effect transistor (GNRFET). Zigzag graphene nanoribbon (ZGNR) is used as semi-metallic drain and source terminal to a pristine armchair graphene nanoribbon (AGNR) that acts as a semiconducting channel. In addition, a single dopant, either nitrogen or boron is added to create lightly-doped drain and source contact. The electronic properties of graphene nanoribbon (GNR) with lightly-doped drain and source contacts are obtained from tight-binding approach. With self-energy matrices, the lightly-doped contacts Hamiltonian matrices are combined with the pristine channel Hamiltonian matrix. The density of states (DOS) are simulated based on the non-equilibrium Green's Function (NEGF) formalism. Our findings are then compared with published research work. Furthermore, it is demonstrated that the DOS of the overall GNR structure still retain a small band gap and possess semiconducting properties when the channel is connected to semi-metallic contact at the drain and source terminal.

Intact Crystalline Semiconducting Graphene Nanoribbons from Unzipping Nitrogen-Doped Carbon Nanotubes.

Unzipping carbon nanotubes (CNTs) may offer a valuable route to synthesize graphene nanoribbon (GNR) structures with semiconducting properties. Unfortunately, currently available unzipping methods commonly rely on a random harsh chemical reaction and thereby cause significant degradation of the crystalline structure and electrical properties of GNRs. Herein, crystalline semiconducting GNRs are achieved by a synergistic, judiciously designed two-step unzipping method for N-doped CNTs (NCNTs). NCNTs are effectively unzipped by damage-minimized, dopant-specific electrochemical unzipping and subsequent sonochemical treatment into long ribbon-like nanostructures with crystalline basal planes. Owing to the nanoscale dimension originating from the dense nucleation of the unzipping reaction at highly NCNTs, the resultant GNRs demonstrate semiconducting properties, which can be exploited for chemiresistor-type gas-sensing devices and many other applications.

Electronic structure, strain effects and transport property of armchair graphene nanoribbon with variously possible edge oxidation

Graphene and related 1D structures have been investigated intensely so far. Here, to consider expanding their potential applications and explore their more underlying physics, we construct armchair graphene nanoribbon models with variously possible edge oxidation, and their geometrical stability, electronic properties, carrier mobility, strain effect, and device properties of are studied systematically, based on the first-principles method. The calculated edge formation energy and molecular dynamics simulations show that these ribbons are quite stable, and the calculated Gibbs free energy suggests that they likely exist in different O-atom concentration environment. Different oxidation leads to different hybridization subbands, making rich and versatile electronic structures, in particular causing various differences in the band gap for ribbons. Large carrier mobility and significant carrier polarization are predicted as well. Under applied strain conditions, some ribbons exhibit flexibly tunable band gaps and the transition of electronic phases, which promises a potential application for developing nanoscale mechanical switches reversibly working between different electronic phases. In addition, the edge oxidation can improve the related ribbon device behavior, such as producing significant negative differential resistance effects. Therefore, the structures proposed here might have important potential applications for developing future on-demand nano devices.

Position and momentum mapping of vibrations in graphene nanostructures.

Propagating atomic vibrational waves-phonons-determine important thermal, mechanical, optoelectronic and transport characteristics of materials. Thus a knowledge of phonon dispersion (that is, the dependence of vibrational energy on momentum) is a key part of our understanding and optimization of a material's behaviour. However, the phonon dispersion of a free-standing monolayer of a two-dimensional material such as graphene, and its local variations, have remained elusive for the past decade because of the experimental limitations of vibrational spectroscopy. Even though electron energyloss spectroscopy (EELS) in transmission has recently been shown to probe local vibrational charge responses1-4, such studies are still limited by momentum space integration due to the focused beam geometry; they are also restricted to polar materials such as boron nitride or oxides1-4, in which huge signals induced by strong dipole moments are present. On the other hand, measurements on graphene performed by inelastic X-ray (neutron) scattering spectroscopy5-7 or EELS in reflection8,9 do not have any spatial resolution and require large microcrystals. Here we provide a new pathway to determine phonon dispersions down to the scale of an individual free-standing graphene monolayer by mapping the distinct vibrational modes for a large momentum transfer. The measured scattering intensities are accurately reproduced and interpreted with density functional perturbation theory10. Additionally, a nanometre-scale mapping of selected momentum-resolved vibrational modes using graphene nanoribbon structures has enabled us to spatially disentangle bulk, edge and surface vibrations. Our results are a proof-of-principle demonstration of the feasibility of studying local vibrational modes in two-dimensional monolayer materials at the nanometre scale.

Multiwavelength Raman spectroscopy of ultranarrow nanoribbons made by solution-mediated bottom-up approach

Here we present a combined experimental and theoretical study of graphene nanoribbons (GNRs), where detailed multi-wavelength Raman measurements are integrated by accurate ab initio simulations. Our study covers several ultra-narrow GNRs, obtained by means of solution-based bottom-up synthetic approach, allowing to rationalize the effect of edge morphology, position and type of functional groups as well as the length on the GNR Raman spectrum. We show that the low-energy region, especially in presence of bulky functional groups is populated by several modes, and a single radial breathing-like mode cannot be identified. In the Raman optical region, we find that, except for the fully-brominated case, all GNRs functionalized at the edges with different side groups show a characteristic dispersion of the D peak (8-22 cm-1/eV). This has been attributed to the internal degrees of freedom of these functional groups, which act as dispersion-activating defects. The G peak shows small to negligible dispersion in most of the cases, with larger values only in presence of poor control of the edges functionalization, exceeding the values reported for highly defected graphene. In conclusion, we have shown that the characteristic dispersion of the G and D peaks offer further insight on the GNR structure and functionalization, by making Raman spectroscopy an important tool for the characterization of GNRs.

The electronic and transport properties of Li-doped graphene nanoribbons: An ab-initio approach

The metal-to-semiconductor transition has been noticed in graphene nanoribbons (GNRs) with various novel electronic and structural characteristics. The prospective and scope of GNRs for an array of implications could be spread significantly by this transition. Based on density functional theory (DFT) calculations, we studied the electronic and transport properties of zig-zag GNRs doped with lithium (Li) along with different edge morphology. Zig-zag nanoribbons are known to exhibit metallic behaviour without using spin. The structural properties, namely, edge state, doping and ribbon width, can be considered to affect the electronic properties of GNR structures. In this study, the changes in the electronic properties by doping a Li atom with various atomic percentages (16.6%, 33.3%, 50% and 66.6%) were investigated. Calculations were done by employing the local density approximation (LDA) based on DFT. In the presence of unique edge states, the edge-modified systems exhibit a noticeable change with prominent and better Li mobility. As a result, it has been observed that substituting two Li atoms at the carbon edges is more predominant compared to other doping configurations. We expect that our peculiar results will have potential applications in energy conversion, solar cells and thermoelectric devices.

Energetics and electronic structure of graphene nanoribbons under uniaxial torsional strain

We investigate the energetics and electronic structure of graphene nanoribbons (GNRs) with structural torsion in terms of their torsion angles and edge shapes using the density functional theory with the generalized gradient approximation. The structural torsion with the period of nanometer scale does not cause the increase of the total energy of GNRs up to the torsion angle of 20°. On the other hand, the total energy with respect to the torsion is weakly dependent on the edge angle. The GNR with zigzag edges is the most flexible against the structural torsion, while those with armchair and chiral are robust. Electronic band structures of GNRs are also insensitive to the structural torsion, because the local atomic structure retains its flat conformation up to the largest torsion.

(Invited) Electronic Structure and Raman Fingerprint of Atomically Precise Graphene Nanoribbons – a Key for Integration in Devices.

Graphene nanoribbons (GNRs) are a novel tunable material that combines the best attributes of graphene and carbon nanotube worlds. We present our recent results on the electronic, optical and vibrational properties of atomically precise GNRs, particularly, pristine and boron-doped armchair graphene nanoribbons of N = 7 carbon atoms width (7-AGNRs and B-7AGNRs). Spatially aligned GNRs are synthesized by bottom-up approach on stepped Au surface. Using angle-resolved photoemission spectroscopy (ARPES) we reveal energy band dispersion of these systems. ARPES data allow us to determine the energies of one-dimensional sub-bands and the effective masses of charge carriers, which are important for charge transport characterization. Using ultra-high vacuum Raman spectroscopy we probe phonons of air-sensitive B-7AGNRs in situ and reveal the existence of characteristic splittings and red-shifts of vibration modes due to the presence of boron atoms. We perform transfer of 7-AGNRs on to the insulating substrate, which allows tuning and probing their optical response. Particularly, bright photoluminescence in 7-AGNRs is controllably induced by defect-engineering. Finally, we partially transform N=7 to narrow-gap N=14 AGNRs via lateral edge fusion on large-area surface. The band structure of 14-AGNRs is visualized by ARPES. Raman fingerprint allows easy identification of type and orientation of GNRs on both metallic and insulating substrates. The fused GNRs are also integrated in the field-effect transistor.

Interface Coupling as a Crucial Factor for Spatial Localization of Electronic States in a Heterojunction of Graphene Nanoribbons

The heterojunction (HJ) band structures of graphene nanoribbons (GNRs), generated by modulating ribbon width, are currently considered for use in tunnel field-effect transistors and optoelectronic applications, but interfacial states often exist and remove the band offsets. Here first-principles calculations show that these states are induced by C---C bonds at the interfacial edge, and can be eliminated by ``spare'' coupling of wide and narrow GNR segments, which is predicted to be stable at room temperature. Knowing how best to construct GNR HJs naturally will promote the technologies based on them.

Structure-dependent electrical properties of graphene nanoribbon devices with graphene electrodes

Graphene nanoribbons (GNRs) are a novel and intriguing class of materials in the field of nanoelectronics, since their properties, solely defined by their width and edge type, are controllable with high precision directly from synthesis. Here we study the correlation between the GNR structure and the corresponding device electrical properties. We investigated a series of field effect devices consisting of a film of armchair GNRs with different structures (namely width and/or length) as the transistor channel, contacted with narrowly spaced graphene sheets as the source-drain electrodes. By analyzing several tens of junctions for each individual GNR type, we observe that the values of the output current display a width-dependent behavior, indicating electronic bandgaps in good agreement with the predicted theoretical values. These results provide insights into the link between the ribbon structure and the device properties, which are fundamental for the development of GNR-based electronics.

On Basic Boolean Function Graphene Nanoribbon Conductance Mapping

In this paper, we augment a trapezoidal Quantum Point Contact topology with top gates to form a butterfly Graphene Nanoribbon (GNR) structure and demonstrate that by adjusting its topology, its conductance map can mirror basic Boolean functions, thus one can use such structures instead of transistors to build carbon-based gates and circuits. We first identify by means of Design Space Exploration specific GNR topologies for 2- and 3-input {AND, NAND, OR, NOR, XOR, XNOR} and demonstrate by means of the Non-Equilibrium Green Function - Landauer based simulations that butterfly GNR-based structures operating at $V_{\text {DD}}=$ 0.2 V outperform 7 nm @ $V_{\text {DD}}=$ 0.7 V CMOS counterparts by 2 to 3, 1 to 2, and 3 to 4, orders of magnitude in terms of delay, power consumption, and power-delay product, respectively, while requiring 2 orders of magnitude less active area. Subsequently, we investigate the effect of $V_{{\text {DD}}}$ variations and the $V_{{\text {DD}}}$ value lower bound. We demonstrate that the NOR butterfly GNR structures are quite robust as their conductance and delay are changing by no more than 2% and 6%, respectively, and that AND and NOR GNR geometries can operate even at 10 mV. Finally, we consider the aspects related to the practical realization of the proposed structures and conclude that even if there are still hurdles on the road ahead the latest graphene fabrication technology developments, e.g., surface-assisted synthesis, our proposal opens an alternative towards effective carbon-based nanoelectronic circuits and applications.

Metal doped armchair graphene nanoribbons: electronic structure, carrier mobility and device properties.

Functionalizing graphene to develop on-demand nanodevices is highly desirable, but still remains challenging. Here, we theoretically propose the functionalization of armchair graphene nanoribbons by low-concentration metal (M) atom (M = Ti, Ni, Sn, or Hg) doping and investigate the structural stability and electronic behaviors of these doped systems in depth. The calculated binding energy and formation energy as well as the molecular dynamics simulation show that the geometries of these hybridized ribbons are rather stable. With metal doping, the ribbons present rich and flexibly tunable bandgaps, depending on the metal atom and doping position, which can be attributed to newly emerged hybridized subbands near the Fermi level and the entire energy band structure shifting upward due to the increased electron number in the ribbon donated from the dopant. These bandgaps can also be further tuned substantially by the stress. And the carrier mobility is calculated based on the deformation potential theory, which shows that the different metal doping can effectively control the carrier mobility, and a large carrier polarity can also be clearly observed. Furthermore, the metal doping can significantly enhance the device properties of the ribbon as compared with those of the pristine ribbon, such as creating a large negative differential resistance phenomenon. These studies demonstrate that these doping systems might hold promising applications in nano-electronics.

Two-dimensional electronic spectroscopy of graphene nanoribbons in organic solution

We unravel the electronic structure of graphene nanoribbons in solution using two-dimensional electronic spectroscopy. We identify different excitons, their vibrational couplings and find that exciton diffusion or relaxation among the exiton sublevels takes place in ~300 fs.

Distinguishing Zigzag and Armchair Edges on Graphene Nanoribbons by X-ray Photoelectron and Raman Spectroscopies.

Graphene nanoribbons (GNRs) have recently emerged as alternative 2D semiconductors owing to their fascinating electronic properties that include tunable band gaps and high charge-carrier mobilities. Identifying the atomic-scale edge structures of GNRs through structural investigations is very important to fully understand the electronic properties of these materials. Herein, we report an atomic-scale analysis of GNRs using simulated X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Tetracene with zigzag edges and chrysene with armchair edges were selected as initial model structures, and their XPS and Raman spectra were analyzed. Structurally expanded nanoribbons based on tetracene and chrysene, in which zigzag and armchair edges were combined in various ratios, were then simulated. The edge structures of chain-shaped nanoribbons composed only of either zigzag edges or armchair edges were distinguishable by XPS and Raman spectroscopy, depending on the edge type. It was also possible to distinguish planar nanoribbons consisting of both zigzag and armchair edges with zigzag/armchair ratios of 4:1 or 1:4, indicating that it is possible to analyze normally synthesized GNRs because their zigzag to armchair edge ratios are usually greater than 4 or less than 0.25. Our study on the precise identification of GNR edge structures by XPS and Raman spectroscopy provides the groundwork for the analysis of GNRs.

Electronic structure of graphene nanoribbons on hexagonal boron nitride

Hexagonal boron nitride is an ideal dielectric to form two-dimensional heterostructures due to the fact that it can be exfoliated to be just few atoms thick and its a very low density of defects. By placing graphene nanoribbons on high quality hexagonal boron nitride it is possible to create ideal quasi one dimensional (1D) systems with very high mobility. The availability of high quality one-dimensional electronic systems is of great interest also given that when in proximity to a superconductor they can be effectively engineered to realize Majorana bound states. In this work we study how a boron nitride substrate affects the electronic properties of graphene nanoribbons. We consider both armchair and zigzag nanoribbons. Our results show that for some stacking configurations the boron nitride can significantly affect the electronic structure of the ribbons. In particular, for zigzag nanoribbons, due to the lock between spin and sublattice degree of freedom at the edges, the hexagonal boron nitride can induce a very strong spin-splitting of the spin polarized, edge sates. We find that such spin-splitting can be as high as 40~meV.

The Investigations of Electronic Structure in Armchair Graphene Nanoribbons Doped B at the Edge

In this paper, the electronic structure in armchair graphene nanoribbons (AGNRs) and graphene nanoribbons doped B at the edge (B-AGNRs) are obtained based on the first principle theory. It shows that the band gap has the oscillation characteristic whose period is 3. The band gap oscillating characteristic gradually vanishes and tends to be stable after doping B at graphene nanoribbons edge. This provides a theoretical guidance for developing the stable graphene nanodevices.

Effect of acoustic phonon scattering on impact ionization rate of electrons in monolayer graphene nanoribbons

In this paper, we have studied the behavior of impact ionization rate of electrons in monolayer graphene nanoribbons due to various scattering mechanisms. The effects have been analyzed using the comprehensive analytical expressions, which result from all the possible inelastic interactions before the ionizing collision inside the graphene nanoribbons structures. These interactions include scattering of electrons due to acoustic phonon, inter-carrier scattering and optical phonon. Also, the obtained results are compared with previously reported data. The overestimation in earlier data has been analyzed in detail through the developed analytical expression. In addition to this, the percentage change in electron ionization rate has been calculated by considering it with and without acoustic phonon scattering.

Unique thermal contraction of zeolite-templated carbons enabling micropore size tailoring and its effects on methane storage

Zeolite-templated carbons (ZTCs) were prepared by the carbon replication of two different zeolite structures, i.e., BEA and FAU. We demonstrated that the micropore size of the ZTCs could be systematically controlled via post-synthesis thermal contraction. The thermal contraction was the unique feature of ZTCs, which was not observed with conventional activated carbons. The ZTCs had extra-high H contents on the abundant carbon edge sites because of their 3-dimensionally connected graphene nanoribbon structure. Thermal treatment in the absence of zeolite templates induced further dehydrogenation and densification of carbon framework, which caused such structural contraction. The ZTCs with tailored micropore sizes (1.1–1.5 nm) were used to study the effects of microporous structures on CH4 adsorption. Notably, the ZTCs with micropores smaller than 1.3 nm showed abnormal increases in the isosteric heat of adsorption with increasing CH4 coverage. This strongly indicated the presence of substantial lateral interactions between the adsorbates within these uniform micropores. The ZTC prepared from BEA zeolite and subsequently treated at 873 K showed the most promising volumetric CH4 storage capacity (210 cm3STP cm−3) and working capacity (175 cm3STP cm−3) at 5–65 bar due to its optimum microporous structures and uniform particle morphology enabling efficient particle packing.

Calculation and Analysis of Graphene Nanoribbons Field Effect Transistor Structure and Characteristics

Because there is no gap between conduction band and valence band of graphene, it is difficult to achieve switching characteristics when transistors are fabricated. In order to open the graphene band gap, researchers have explored many methods, such as tailoring Shi Mocheng quantum dots, nanobelts, nano grids or laying graphene on a special substrate, one of the feasible ways is to regulate the electrical properties of graphene by doping. Through the analysis on the structure and properties of graphene band calculation, with the help of computer aided simulation, the ID and VG characteristics analysis of the Schottky barrier graphene field effect transistor (SBFET) and MOS structure of graphene field effect transistor (MOSFET) are investigated in this study. The results are of high reference value for the study of the structure and properties of graphene nanoribbons.